The Fischer Tropsch process is often used for the conversion of hydrocarbonaceous feedstocks into normally liquid and/or normally solid hydrocarbons. The feed stock (e.g. natural gas, associated gas, coal-bed methane residual hydrocarbon fractions, biomass and/or coal) is converted in a first step into a mixture of mainly hydrogen and carbon monoxide (this mixture is often referred to as synthesis gas or syngas). The partial oxidation of gaseous feedstocks, producing mixtures of especially carbon monoxide and hydrogen, can take place according to various established processes. These processes include the Shell Gasification Process, a process in which e.g. natural gas is reacted with an oxygen containing gas in the absence of a catalyst (partial oxidation). A comprehensive survey of this process can be found in the Oil and Gas Journal, Sep. 6, 1971, pp 86-90. Gasified coal which includes a mixture of H2 and CO may also be used. Another process to produce synthesis gas is steam reforming. Combinations are also possible.
The synthesis gas is then converted in a second step over a suitable catalyst at elevated temperature and pressure into paraffinic compounds ranging from methane to high molecular weight molecules comprising up to 200 carbon atoms, or, under particular circumstances, even more.
Four main products streams are recovered from a Fischer-Tropsch reaction: a waxy hydrocarbon stream, a gaseous stream which on cooling results in light hydrocarbons, impure water and off-gas. The off-gas comprises unconverted synthesis gas, C2-C4 olefins, light C1-C4 hydrocarbons, CO2 and inerts such as N2 and Ar. Flaring the off-gas is inefficient and damaging to the environment. The use of the off-gas as fuel is also less efficient.
It has been proposed in WO03/035590 to recirculate the off-gas (referred to therein as tail gas) back into the Fischer-Tropsch reactor or into gasifiers. Gasifiers produce CO and H2 for onward reaction in the Fischer-Tropsch reaction.
Whilst some of the off-gas can be recirculated to the Fischer-Tropsch reactor, recirculation of all of the off-gas to the Fischer-Tropsch reactor causes CO2, CH4 and inerts to build up thus reducing the amount of hydrocarbons produced by the Fischer-Tropsch reactor.
Feeding the off-gas back to the gasifiers (which produce the H2 and CO mixture) results in problems relating to the relative ratio of H2/CO. Gasifiers typically produce a H2/CO ratio of 1.7-1.8 whereas the Fischer-Tropsch reaction typically requires a H2/CO ratio of around 2.05. The recirculation of off-gas to the gasifiers reduces the H2/CO ratio away from the preferred 2.05 ratio.
The third option disclosed in WO03/035590 is to use the off-gas as a fuel for power generation. However, in practice this may not consume all of the available off-gas and so an alternative use for the off-gas still needs to be found.